/*
 * @lc app=leetcode.cn id=102 lang=java
 *
 * [102] 二叉树的层序遍历
 */

// @lc code=start

import java.util.Deque;
import java.util.LinkedList;
import java.util.Queue;
import java.util.List;
import java.util.ArrayList;

import TreeNode;

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */




class Solution {

    public List<List<Integer>> levelOrder(TreeNode root) {

        Queue<TreeNode> queue = new LinkedList<>();

        List<List<Integer>> res = new ArrayList<>();

        if (root != null) queue.add(root);

        while (!queue.isEmpty()) {

            List<Integer> tmp = new ArrayList<>();

            for(int i = queue.size(); i > 0; i--) {

                TreeNode node = queue.poll();
                tmp.add(node.val);

                if (node.left != null) queue.add(node.left);
                if (node.right != null) queue.add(node.right);
            }
            
            res.add(tmp);
        }
        return res;
        
    }
}
// @lc code=end

